Nuclear Magnetic Resonance Imaging (MRI) is a well known diagnostic procedure which is extensively used in the medical field to noninvasively image internal biological tissue. As is also well known, MRI relies on the nuclear magnetic resonance (NMR) of nuclei, and the fact that when tissue nuclei are placed in the environment of a strong external magnetic field they will each assume a discrete energy state. When considered together, it is the statistical distribution of those energy states of the atomic nuclei in a magnetic field that leads to a macroscopic magnetic moment. Importantly for MRI, these nuclear magnetic moments react to a particular radio frequency (RF) radiation by specifically changing their orientations.
NMR takes advantage of the fact that while tissue nuclei are in an external magnetic field, induced changes in their energy states will generate signals which are characteristic of the tissue. In particular, with appropriate changes in orientation of the magnetic moments, it is possible to generate spin echo signals. Not surprisingly, many variables are involved in this process, and importantly, they are interrelated.
As indicated above, nuclei assume orientations in a magnetic field which may be subsequently changed by RF radiation. The RF radiation which is most effective for inducing such a change has a particular frequency which depends on the magnitude of the magnetic field at the location of the nucleus. This particular frequency, more familiarly known as the Larmor frequency, is equal to the angular frequency of precession of the nucleus spin vector about the direction of the magnetic field vector. Generally stated, it is necessary to irradiate nuclei with their particular Larmor frequency to induce rotations of the net magnetic moment vector. Particularly important to the present invention is the fact that RF pulses which cause one hundred and eighty degree (180.degree.) rotations of the magnetic moment vectors are capable of generating spin echo signals by refocussing a large number of magnetic moments having a range of Larmor frequencies. This fact is of less importance when NMR is conventionally performed using nearly homogeneous fields and the range of Larmor frequencies is small. The present invention, however, is concerned with inhomogeneous magnetic fields wherein the range of Larmor frequencies may be quite large due to the use of a remotely positioned imaging device.
Unlike homogeneous magnetic fields, it is an inherent characteristic of inhomogeneous magnetic fields that they have a field strength gradient (G.sub.z) which dominates the behavior of nuclei in the magnetic field. Insofar as MRI is concerned, there are three phenomena which can be directly attributed to the presence of a field strength gradient. For an effective MRI procedure, these phenomena must dealt with. They are: 1) the requirement for increased bandwidth in the RF radiation; 2) the fact that the nuclei will defocus immediately after being pulsed with the RF radiation; and 3) the tendency for defocussing nuclei to diffuse.
The requirement for an increased bandwidth of RF radiation arises because, in the presence of a field strength gradient, each nucleus will have a slightly different Larmor frequency. The consequence of this is that, as more nuclei are to be influenced during an MRI procedure, the bandwidth of the RF radiation must necessarily be broadened to include all of the required Larmor frequencies. It happens, a broadened bandwidth allows additional noise to pass. This lowers the signal-to-noise ratio (SNR) and makes detection of the spin echoes more uncertain.
Insofar as defocussing is concerned, it is well known that inmediately after the magnetic moments of nuclei in a magnetic field have been reoriented by a pulse of RF radiation, the field strength gradient (G.sub.z) will cause the magnetic moments to begin defocussing. An adverse consequence of this is that as the nuclei defocus, they loose their coherence. The result is that the net magnetic moment is weakened and becomes less detectable. Consistent with the aforementioned wide range of Larmor frequencies, the time duration for this loss of coherence is significantly shortened in inhomogeneous field NMR. Correspondingly, the time to refocus a wider range of Larmor frequencies is also shortened.
Defocussing magnetic moments are also subject to diffusion. This is the well known random displacement of particles that occurs when there are variations in particle concentration.
With the difficulties imposed by the above stated phenomena, it has been recognized that a process of continually refocussing spin echoes at an accelerated rate is necessary for effective NMR in an inhomogeneous magnetic field. In particular, it has been recognized that by performing accelerated refocussing at a rate which is proportional to G.sub.z, diffusion can be controlled and the SNR can be improved through the use of averaging techniques. For an example of a device which discloses and incorporates such a process, see U.S. application Ser. No. 08/012,053, filed on Feb. 1, 1993, for an invention entitled "Remote Positioned MRI System" which is assigned to the same assignee as the present invention and which is incorporated herein by reference.
As is well known, in order for an MRI system to make images, it must also incorporate an encoding procedure. This is required in order to generate the different patterns of net magnetic moments which can be later processed to create particular pixels for the resultant image. Typically, encoding is done by arbitrarily advancing or delaying the phases of the magnetic moments of the nuclei using x and y gradient pulses.
In Conventional homogeneous field MRI systems, well known combinations of phase encoding and frequency encoding are used. Techniques for encoding in inhomogeneous field MRI have also been disclosed in the art. For example, U.S. Pat. No. 4, 656,452, which issued to Bendel for an invention entitled "Method to Eliminate the Effects of Magnetic Field Inhomogeneity in NMR Imaging and Apparatus Therefor", describes a technique of interleaving gradient pulses within a pulse train of refocussed spin echoes. According to Bendel, this interleaving is accomplished in such a manner that the encoding patterns continue to accumulate while the defocussing caused by field inhomogeneities are refocussed. Bendel does not, however, specifically address the SNR and diffusion issues encountered when a remotely positioned device is used to generate an inhomogeneous field with a dominant G.sub.z. Other difficulties with interleaved gradient pulses arise due to the wide ranges of Larmor frequency offset and RF amplitude offset. More specifically, within any coordinate system the nuclear magnetization will have an x component (M.sub.x) and a y component (M.sub.y). It is well known in the art that depending on the phase of the refocussing pulses, the contribution to the spin echo from one of these components of the nuclear magnetization may be lost. This problem is exacerbated when gradient pulses are interleaved within the refocussing pulse train as suggested by Bendel. This is so because each subsequent gradient further rotates magnetization components into the direction where they are not refocessed.
It has been recognized in the art that specific combinations of refocus pulse phases may be used in an attempt to preserve both components of the encoded magnetizations during the refocussing process. For example, in an article entitled "Modified Carr Purcell Meiboom Gill Sequence for NMR Fourier Imaging Applications" which appeared in the Journal of Magnetic Resonance, Vol. 69, pp. 488-489 (1986), Maudsley describes the use of a phase pattern for a group of four refocussing pulses. Further stabilization of both components of encoded magnetization is afforded by a different phase pattern used in groups of eight refocussing pulses, as described in the previously cited application No. 081,012,053. With this stabilization of both components, some of the gradient pulses may be selectively interleaved within a train of 180.degree. refocussing pulses.
It is nevertheless recognized for the present invention that the ability of composite pulse patterns to fully refocus both components may still be impaired after large numbers of refocussing pulses, e.g. greater than one hundred pulses. It is further recognized, that since one of the components can be completely preserved over the course of hundreds of refocussing pulses, the entire pattern of encoded moments may be obtained by a two step procedure. In this two step procedure, first one component of magnetization for a given encode is preserved, and in the second step, the other component is preserved. It is appreciated that this is possible provided the gradient pulses are not interleaved within the refocussing pulse train, and the phase of the refocussing pulses is adjusted according to which component is to be preserved. To do this, it is also appreciated that each gradient encoding pattern must be imparted twice, once for each of the components that is preserved.
Despite the fact that two separate steps are required to obtain the complete measurement for a given encoding pattern, the proposed method nevertheless effectively allows a single encoded measurement to be obtained from thousands of accelerated refocussed spin echoes.
Some additional considerations which are pertinent to MRI procedures wherein inhomogeneous magnetic fields are used, should be noted. The time period during which the train of spin echoes are recorded is important and has an effect on the spin echo signals. Specifically, for a nuclei in a magnetic field there are two time periods which are characteristic of their magnetic moments. The first time period of interest is the time required for the nuclei to restore their equilibrium after they have been tilted by an initial 90.degree. pulse. This time period (T.sub.1), also commonly referred to as the spin-lattice time, is the time for longitudinal magnetization of the nuclei to be restored. The other time period of interest is the time over which components of the magnetic moments decay after they have been tilted. This second time period (T.sub.2) is commonly referred to as the spin-spin relaxation time. T.sub.1 is not equal to T.sub.2. In many instances, T.sub.1 is approximately one order of magnitude (10X) greater than T.sub.2. The importance of T.sub.2 for MRI is that the refocussing and recording of nuclear magnetic moments must be accomplished during T.sub.2. However, the time for recording can be varied within the time period T.sub.2 and, depending on how much of T.sub.2 is taken to retrieve spin echo signals, the net magnetic moment measurement will be either T.sub.1 weighted or T.sub.2 weighted.
In light of the above, it is an object of the present invention to provide a method for maintaining encoded component coherence of nuclear magnetic moments in a material to be imaged in an inhomogeneous magnetic field when using a remotely positioned MRI device, which avoids intermixing the encoding function and the refocussing function. Another object of the present invention is to provide a two step method for maintaining encoded component coherence in a material to be imaged in an inhomogeneous magnetic field when using a remotely positioned MRI device, which uses only one type of 180.degree. refocussing pulses during each step in order to preserve all of a same component of the encoded magnetic moment profile for recording spin echoes. Yet another object of the present invention is to provide for a two step method for maintaining encoded component coherence of a material to be imaged in an inhomogeneous magnetic field when using a remotely positioned MRI device which alternates the type of 180.degree. pulses in sequential refocussing and recording episodes to obtain spin echoes for both the x and the y components of the magnetic moment. Still another object of the present invention is to provide for a two step method for maintaining encoded coherence of nuclei in a material to be imaged in an inhomogeneous magnetic field when using a remotely positioned MRI device, which permits variation in the duration of refocussing pulse train function in order to permit T.sub.1 or T.sub.2 weighted net magnetic moment measurements. Yet another object of the present invention is to provide a two step method for maintaining encoded coherence of nuclei in a material to be imaged in an inhomogeneous magnetic field when using a remotely positioned MRI device, which is easy to use, simple to implement, an comparatively cost effective.